Don't log exceptions for obviously incorrect stream tokens

[erikjohnston]

We log incorrect ones as we want to catch bugs where Synapse returns bad tokens. However, sometimes clients just send tokens that are e.g. empty.

[MadLittleMods]

Clarified "at the exception-level" because this change is a bit confusing otherwise (we still log and raise and exception, just not at the exception-level for obviously wrong tokens).

[MadLittleMods]

Perhaps we should think about just ignoring these obviously wrong tokens altogether or log at a much lower level.

Suggested change

	# Check it looks like a Synapse token first. We do this so that
	# we don't log exceptions for obviously incorrect tokens.
	if not string or string[0] not in ("s", "t", "m"):
	logger.warning("Invalid token %r", string)
	raise SynapseError(400, f"Invalid room stream token {string:!r}")
	# Check it looks like a Synapse token first. We do this so that
	# we don't log exceptions for obviously incorrect tokens.
	if not string or string[0] not in ("s", "t", "m"):
	logger.debug("Invalid token %r", string)
	raise SynapseError(400, f"Invalid room stream token {string:!r}")

[erikjohnston]

Yeah, I was going to suggest at least only logging the empty string case as debug. It's plausible that a non-empty token is from a future synapse (after upgrade/downgrade) or something. However, I realised we'll log these tokens anyway when we return the 400 (as we log the error string), so actually adding an extra log line here is pointless, so I've removed it entirely.